Melting-points of Aluminium, Silver, Gold, Copper, Sfc. 43 



was experienced with this arrangement with silver, gold, or 

 copper. With aluminium, however, a peculiar action occurred, 

 the cause of which in the time available for investigation 

 could not be determined beyond doubt. The phenomenon 

 was that after a few minutes of constant temperature at the 

 melting-point, the indication of .the thermo-couple fell off with 

 increasing rapidity. On withdrawing the couple, cleaning it, 

 or clipping it off and restoring it to place, the melting metal 

 meanwhile being untouched, the indications returned to their 

 original high value. The apparent explanation was the for- 

 mation of a slag between the wires ; but this was not entirely 

 satisfactory. The use of a plumbago crucible in place of the 

 clay and an entirely fresh lot of aluminium did not remove 

 the phenomenon, and gave the same initial readings, which, 

 it could not be doubted, were the ones corresponding to the 

 melting-point. The fusion of the aluminium was, however, 

 the least sharply defined of all the metals used. 



The fusion of platinum was, of course, differently effected. 

 For this the two wires of the couple were laid close together 

 on a piece of lime. An oxybydrogen flame was then directed 

 upon their ends and the platinum fused into a globule which 

 with care could be made to travel slowly up the wire. There 

 was no difficulty in obtaining steady temperatures for a suffi- 

 cient period to make the necessary readings, and check results 

 to 01 per cent, were obtained on different days. 



The galvanometer, keys, coils, and all junctions of dis- 

 similar metals, were, so far as possible, covered with boxes 

 of wood, pasteboard, or asbestos to maintain uniformity of 

 temperature, and thus minimize local thermo-electric disturb- 

 ances. With this precaution, no sensible trouble from that 

 source was experienced. 



The procedure is as follows : — To take the observation for 

 vapour of sulphur, for instance, the hot and cold junctions 

 are exposed as described. After a sufficient time the main 

 circuit is closed, the thermal circuit is connected to a suitable 

 part of a, b, c, d, and the rheostats W are adjusted until on 

 pressing the key no deflexion occurs in the galvanometer Gr. 

 At this instant A is read, which gives the current c in the 

 main circuit. The adjustment is disturbed and remade a 

 number of times, and the resulting readings should check to 

 the nearest tenth of a division of A, provided the metal has 

 reached a steady state of temperature. 



By this adjustment the drop of potential cr due to the 

 current c amperes in the part r ohms of the resistance a, />, c. d, 

 spanned by the thermal circuit is made equal to the total re- 

 sultant E.M.F. in the thermal circuit. The latter, which will be 



